Typical cathode ray tubes include a source of electron beams which scan a face plate or screen. As is known, if the scan lines traverse the screen at constant speed, the display on the screen will not be linear. This is due to the fact that, as the electron beam from the electron gum scans the screen, it is functioning in an arc of a circle; and the screen is generally not formed in the same arc as the swing of the electron beam but, rather, the screen tends to be more planar in shape. Hence, the electron beam will be traveling for a shorter distance across the face at the center of the screen as compared to the distance that the beam travels across the face at either edge of the screen. Accordingly, adjustment of the rate of speed of the beam as it moves across the screen must be made in order to obtain linearity of display.
As will be explained hereinbelow, the present invention is directed to the provision of a variable magnetically biased linearity control for compensating for the foregoing variations to thereby provide a more linear display.
The linearity of display is of interest with respect to television picture tubes. However, the linearity of the display becomes critical in other more demanding applications, such as for computer terminals and word-processing terminals, wherein alpha-numeric displays including, for example, typewritten pages, are displayed on the screen. If the display is non-linear, the digits or type on one portion of the screen may be smaller than the digits or type on another portion of the screen and difficult to read which is, of course, undesirable.
The prior art discloses tuning coils having a dynamic adjustment, connected electrically in series with the deflection yoke of a cathode ray tube. The present invention discloses an improvement over the prior art in providing a tuning coil which is of a simplified construction and which is conventionally and readily adjustable.